Mobile digital computing devices or (“mobile computing devices”) such as laptop computers, personal digital assistants (PDAs), MP3 players, video recorders, cameras and/or other mobile computing devices are steadily increasing in popularity. Laptop computers in particular offer increased mobility as compared to desktop computers. Laptop computers are powered by a battery pack that typically includes batteries that are connected in series. Battery life between charging is one important design consideration. Laptop weight and size are also important design considerations.
Advancements in semiconductor technology are leading to both longer battery life and improved laptop performance. Despite recent improvements in battery life, conventional battery systems used in laptop computers and other mobile computing devices do not efficiently make use of the maximum available energy from the battery pack.
Due to customer preferences, increasing battery life by increasing battery size and/or weight are generally not acceptable. Therefore, battery efficiency should be increased to increase overall battery life. Improvements in battery efficiency are typically accomplished by increasing battery supply voltage. Higher battery voltage is typically accomplished by connecting additional batteries in series and/or increasing the voltage of each battery. Generally, higher battery supply voltage increases efficiency by reducing operating current, which reduces I2R loss or losses due to parasitic distribution resistance. Typical laptop computers operate using a battery voltage between 10.8V nominal (12.6V maximum) and 18V nominal (21V maximum). For example, a stack of three to five lithium ion (LiOn) batteries that are connected in series may be used as a voltage source in laptop computers.
Energy loss due to parasitic distribution resistance is not the sole cause of battery inefficiency in laptop computers and other mobile computing devices. In particular, conversion loss also occurs when converting high battery voltages to voltages that are low enough for use by certain types of semiconductor devices. Conversion loss is also a significant cause of battery inefficiency. For example, a DC/DC converter that converts 12V battery voltage to 1V supply voltage (i.e. 12:1) may have a conversion loss of approximately 12-25%. A DC/DC converter that converts a 4V battery voltage to a 1V supply voltage (i.e. 4:1) may have a conversion loss of approximately 5-10%.